The field of the invention comprises apparatus and a method for reproducing an original image on an electrophotographic member by scanning the elements or information forming structure of the original image and imaging the electrophotographic member with a radiant energy source such as a laser, the imaged electrophotographic member carrying the reproduced original image being thereafter used primarily for printing.
In the case of lithographic offset printing the imaged member is treated to render toned and untoned parts hydrophobic and hydrophilic, respectively and the member comprises the printing plate without further processing. In other cases the toned electrophotographic member may be used as an information source by reading the images or projecting them if transparent or photographically reproducing them if desired. The preferred use of the invention is to make printing plates either on transparent synthetic resin sheeting such as polyester or upon metal such as stainless steel. Each of these substrates is coated with a type of photoconductive coating which will be described hereinafter.
In photography the film exposed to light carries silver salts, which are light sensitive, in a suitable emulsion. Exposure of the film to a scene changes the chemical composition of the salts, there being a precipitation of silver grains as a result of the exposure and the amount of silver precipitated being proportional to the degree of exposure. The film is then developed to render the precipitated silver grains visible. Since the grains are very small, the scene of varying light or continuous tones appears to be reproduced on the film although in fact it is made up of these tiny grains of different size irregularly spaced apart and distributed in accordance with the light distribution of the scene.
In relation to the invention herein, the individual silver grains may be considered as individual information forming structures whose size and distribution determine the overall result being achieved.
In order to print photographs having continuous tones, the so-called halftone process was developed many years ago, which creates an optical illusion in which the tones are represented by patterns of small dots of different sizes printed with ink uniformly applied. Areas to be printed in a dark tone may have large dots closely spaced and possibly overlapping while areas to be printed in a light tone may have small dots widely spaced. As in photography, the eye integrates the patterns of dots together to achieve a reproduction of the represented tones.
Halftone processing forms the dots by photographing the original image through a screen of parallel and perpendicular lines, the resulting photograph having large dots corresponding to the dark areas of the original image and small dots corresponding to the light areas of the original image. Black and white graphics are photographed through the screen once while color graphics are photographed through the screen once for each color to be printed with the screen being rotated to a different angle for each color photograph taken to avoid moire patterning and a different color filter being used to separate the original image into its primary colors for printing.
It should be noted that the dots formed in the halftone process in many cases can be seen with the naked eye, their size generally being much greater than the size of the precipitated silver crystals in an exposed photographic film.
Using other photographic processes, these arrays of spaced dots are then transferred to metal surfaces to form the printing plates which will be installed in the printing press which is to reproduce the original image. For color the printing plates are required to print the respective colored images in precise registration on the receptor which is normally a web of paper and hence there will be as many impressions on a given area of the paper as there are color plates. The composite of these arrays of dots will produce a resultant array of dots many of which will overlie one another to give a color mixture attempting to reproduce the color of the originally photographed image as closely as possible.
In the case of black and white printing the use of the halftone process provides arrays of dots on a receptor or paper which give varying shades of gray between white where there are no dots and black where the dots are so close together and so large that they carry heavy coatings of ink in the press. In the case of color printing, multiple impressions on a receptor are required not only to provide the different shades of light and dark for information content but also to provide the multiple hues of color that are needed to attempt to reproduce the original continuous tone image.
The process of making plates using the same techniques as have conventionally been used, that is making the color separations and deriving the metal plates therefrom has in recent times been effectuated electronically. Photosensors are used to sense the intensity or density of incremental areas of continuous tone original images through colored filters. The sensed densities are converted into digital data which are intended to represent the various densities of the incremental areas of the original image. This digital data is then used to reproduce the original image as an array of dots of uniform area on a heat or light sensitive member from which the printing plate is made.
The sensed densities of the incremental areas of the original image in each color separation are usually treated as steps of a gray scale, the scale extending from the least dense or white to the most dense or black. When the sensed densities are converted to digital data, the sensed densities are converted to digital numbers corresponding to the sensed step of the gray scale. The digital numbers then are used to form a particular pattern of halftone dots on the member from which the plate is made. Each pattern of dots is equivalent in area density to the sensed density of a corresponding incremental area of the original image. When the pattern is printed, theoretically an equivalent area density of ink of each color is transferred to the receptor or paper stock.
It should be noted that the dots formed in making the manual and electronic halftone color separations are different. The dots formed in the manual process vary in surface area and spacing from the surrounding dots to produce the varying densities or shades of gray. Thus as has been explained earlier, a light gray or weak density image is represented by small dots spaced a great distance from the surrounding dots. A dark gray or strong intensity image is represented by large dots almost or actually touching each other. The dots formed electronically are generally fixed in size and spacing. Their size is usually determined by the material used and may be equal in size to the smallest dot formed in the manual process. The varying intensities are formed or represented by the number of dots in a matrix of unit area. Thus a light gray image is represented by a small number of dots in each matrix or in one of several matrices. A dark gray image is represented by a large number of dots in a single matrix.
The speed at which such electronic systems may sense the original image and form or image the halftone array of dots is limited, in part, by the computation times required at both the sensor and imager in converting to and from the digital data. At the sensor some type of conversion must be made from the analog density sensed in the original image to the digital numbers representing the steps of the gray scale. At the imager a conversion must be made from the digital data to the number of dots which are in a matrix or an incremental area of the member from which the printing plate is to be formed. If processing of the data before imaging is to occur to perform tasks such as line or edge enhancement, further time is required and the reproduction process is slowed further.
Another part of such electronic reproduction systems which limits the system speed is the imaged member from which the plate is made. These members generally have been heat or light sensitive members which are imaged with a source of radiant energy such as a laser. The materials from which these members have been made must be exposed or activated with a discrete amount of radiant energy on an elemental area over a certain period of time so as to form the image but not to burn through the material. This poses problems in performing the imaging and increasing the system speed. Imaging in this way may not be as time or labor consuming as manual production of plates but is not rapid enough to perform high speed or on-line imaging. Accordingly it is not utilized to any great extent for the manufacture of printing plates.
The present invention eliminates these speed problems by binarily reproducing the original image which may have continuous tones therein. An incremental area of the original image is sensed to statistically determine whether an information structure is present therein. A binary bit is produced which carries the result of the determination to an imaging apparatus. In the imager a binary element is formed on an electrophotographic member in a sense corresponding to the binary data bit and in a position corresponding to the sensed incremental area. Generally speaking, individual ones of the information structures of the original image are reproduced on the electrophotographic member.
The size and placement of the elements so formed on the electrophotographic member is finer and the speed at which imaging occurs is faster than anything utilizing dots in the prior art. This is feasible because primarily there is a photoconductive coating described and claimed in U.S. Pat. No. 4,025,339 for which the invention is especially advantageous which has a resolution greater than other photoconductive coatings thereby enabling very small undischarged areas on its surface to be toned. These will form the printing elements or formations of the printing plate carrying ink to the receptor. This enables the use of a large number of elements with which to reproduce the original image without any detectable degradation of the resolution of the reproduced image. This particular photoconductive coating also has a discharge rate greater than other photoconductive coatings, enabling a very fast formation of elements on its surface such as by discharging incremental areas of the charged member which allows on-line, high speed reproduction of the original image on the imaged member by discharging incremental areas of the member as fast as the original image can be sensed.
U.S. Pat. No. 3,604,846 discloses a method of sensing an original image and reproducing it on a receptor by means of marking dots. Incremental areas of the original image are sensed by passing a beam of light through the transparent original image and converting the modulated light into electrical signals in a photomultiplier tube. These electrical signals are recorded on a magnetic tape and fed into an electronic computer where they are manipulated into digital data representing the steps of a 10 step gray scale. Each item of data is arranged to represent an incremental area of the original image in a 3.times.3 format, providing 10 density values corresponding to the number of the 9 possible points in the 3.times.3 grid which are blackened by the marking dots formed on the receptor. From the data produced by the computer, a marking device such as an ink jet printer forms the marking dots on the receptor. This patent is an example of a reproduction system utilizing dots formed in a matrix which uses the steps of a gray scale to represent the variations in tone of the original image. This system is not as fast as that enabled by the present invention because of the required computation time between sensing the imaging and does not provide the resolution of the reproduced image as does the present invention.